Gone are the days when illumination in image processing was understood as "light to illuminate the image". Modern lighting components have become high-tech products in which they combine the state-of-the-art knowledge of lighting, electronics, thermodynamics, material engineering and production technology.
The variety of light sources used in image processing has been considerably reduced in recent years due to LED's. However, many other sources are being used:
The homogeneous lamps:
Bright, including spectrum, for continuous operation only, short life, slow switching, vibration sensitive, low efficiency, usually integrated into the cold light source.
Metal halide lamps:
They are very bright, only for continuous, non-universal spectrum operation.
Good for flash (including short sequence), very bright high voltage operation, problems with electromagnetic tolerance, elaborate controls.
Cost-effective (including large area lighting), operation only with high frequency ballast, highly temperature-dependent form inflexible.
Many configuration options for structured lighting, monochromatic lighting, differences in intensity (speckles) make analysis more difficult.
By converting standard lighting into image processing, they are:
Highly proven in the industry, it can be controlled quickly, more and more efficiently, more and more wavelengths/colours/colour combinations, without maintenance or repair.
The light output used by the lighting components is determined by their internal structure. For the various applications in image processing, the direction of light output must be manipulated in a focused manner, using optical components, of the lighting components in order to achieve the desired light effect. Depending on the use, the lighting is formed as:
● Diffuse lighting with the most uniform distribution, in different directions.
● Directed lighting, which has a pronounced preferential direction.
● Telecentric lighting, which is particularly directed and in which the main parallel beams provide most of the light.
● Structured lighting, which has a light distribution structure as a local property of the additional lighting.
The lighting control is an essential part of the lighting, the interface to the controls or the image processing system. Efficient, powerful and reliable image processing can only be achieved by defining lighting scenarios, as these are achieved using control circuits that are specially adapted to the light sources.
A large input power supply (usually 10 to 30 VDC) is an essential quality for the connection of unregulated supply networks (PLC supply voltage).
On the other hand, a constant current source must be integrated. This counteracts the effect of aging on long-term LED indicators, and can provide uniform illumination in the case of short-term switching operations.
Even short-term overheating of the chips leads to extreme and irreversible ageing. Losses in brightness (>50%) and increased failure rates that occur only after years of cold operation can occur in as little as a few hours. Therefore, temperature management is an absolute necessity for LED lighting with constantly increasing performance levels. Directed and based on the heat dissipation design also ensures reliable and non-overheating operation even at high ambient temperatures.
As well as the temporarily stable lighting operation, the ability to control its brightness is an important feature. This can be achieved in several ways:
● Manually by potentiometer.
● Via the analog control voltage (typically 0 to 10 VDC).
● Through the digital interface using a controller.
Adaptive lighting allows for temporary and local brightness, which will be set for complete LED fields and adjusted to the geometry of the material to be inspected and the environment in real time via Ethernet.
The simplest mode of operation is constant lighting. Once switched on, the lights provide illumination for a long period of time.
Lights that do not provide constant illumination function as pulse lighting. They have a quick switch input, through which the lights can be switched on and off with a PLC caliper with a delay time of < 1 ms. This quick change option is always used when complex lighting scenarios require the camera to capture multiple consecutive images under different lighting conditions.
Lights that are only intended for constant illumination are not suitable for pulse operation. Extensive capacitive wiring of the stabilization circuit means that there are long on/off delays that prevent the necessary rapid reaction.
TTL and PLC are typical for activating flash components. Therefore, it is possible to activate the flash, either from the image processing system or the machine controls (PLC).
Once the trigger signal is sent, there is a delay time until the flash is activated. The delay time should be very short so that the material to be inspected is not too long outside the camera's field of view. Typical delay times are about 500 ns.
The maximum flash frequency denotes the highest number of flashes that a flash component can carry. It is very dependent on the quality of the control circuit for the flash illumination.
Flash times are usually between 1 and 100 ms. The flash time required for a specific application depends on a number of factors:
● Speed of the material to be inspected.
● Size of the visual field.
● Maximum motion blur allowed in the image.
● Flash illumination output.
The service life of the lighting depends on the type of light source and other environmental and operating conditions. It can be between 300 hours (powerful halogen lights) and 50,000 hours for LED lighting.
The much quoted MTBF (Mean Time Between Failures) LED illumination of 100,000 hours is for a single red LED. For other wavelengths (blue and UV) they are lower.
In the case of LED flash lighting, it can be assumed that several million flashes can be realized at maximum illumination and without aging, therefore the loss of brightness in certain principles of switching technology is met during construction.
Lighting that is technically the best becomes useless if it is not well defined, and is not permanent, stable and combined.
Standardized perforated grilles are required for clamping, pressure gauges with standardized connection for lighting, suitable for series production in mechanical engineering work.
Like all image processing components, the lighting is also subject to oscillations and vibrations. As a light source, LEDs are extremely shock-resistant, however, the printed circuit board interface or LED bracket is often unable to cope with the force that is produced.